Due to the resistive nature of wide bandgap materials, AlXGa1-XN/GaN HFETs have historically suffered from high contact and access resistance, degrading high-frequency and power performance of these devices. Typically, ohmic contacts are made by annealing a Ti/Al-containing metal stack in order to drive metal through the wide bandgap Al-containing Barrier layer and contact the two-dimensional electron gas (2DEG) below. However, this approach has the disadvantages of inconsistent contact resistance, rough metal morphologies, and a lack of flexibility in device design. Ohmic contact regrowth, in which contact material is regrown after processing steps including masking and/or etching, has been explored as an alternative to this technique with limited success [1-4]. If successful, some advantages of ohmic contact regrowth would include: possible etching of the wide bandgap Al-containing Barrier layer in the contact regions, selective growth of bandgap-engineered or highly-doped material for ultra-low contact resistance and electric field profile modification, greater flexibility in device design and scaling such as reducing the effective source-drain distance for improved high-frequency performance, and the possibility of using non-alloyed metal contact stacks.
In “Low-resistance Ohmic contacts for high-power GaN field-effect transistors obtained by selective area growth using plasma-assisted molecular beam epitaxy”, by S. J. Hong and K. Kim, (Appl. Phys. Lett. 89 042101 (2006)), the authors describe using selective area growth of a n+ doped GaN cap layer on the channel layer as a way to reduce the contact resistance. The present technology differs from Hong, et. al. in that Hong does not regrow the n+GaN on top of the barrier layer in the gate region. As a result, there is no bandgap engineering of the 2DEG below the exposed barrier layer near the gate.
In “High-transconductance self-aligned AlGaN/GaN modulation-doped field-effect transistors with regrown ohmic contacts”, by C. H. Chen, S. Keller, G. Parish, R. Vetury, P. Kozokoy, E. L. Hu, S. P. DenBaars, and U. K. Mishra, (Appl. Phys. Lett. 73 3147 (1998)) the authors regrew n+GaN on the exposed channel layer but not on top of the barrier layer as in the present technology.
Devices processed using the previous approaches, in which n+GaN was regrown in the contact regions generally resulted in relatively high contact and access resistances. An annealing step subsequent to disposing the source or drain contact will not substantially affect the resistance between the source/drain and the channel layer.